1. Field of the Invention
This invention relates to a method of turning on and off the display of an electrochromic display device, and more particularly to a method of turning off the display of a diffusion type (hereinafter described in detail) electrochromic display device.
2. Description of the Prior Art
It has been known in the art that an electrochromic compound presents different colors according to its state, whether it is oxidized or reduced. This is because the spectroscopic absorption is different according to its state. The electroscopic compounds are divided into inorganic compounds and organic compounds. As disclosed in U.S. Pat. No. 3,827,784, various metal oxides are known as the inorganic electrochromic compounds such as tungsten oxide (WO.sub.3) and molybdenum oxide (MoO.sub.3). As the organic electrochromic compounds are known pyridine compounds and aminoquinone compounds for instance, as disclosed in U.S. Pat. No. 3,862,842 and J. Electrochem. Soc. Vol. 121, No. 12, P1555-1558. The inorganic electrochromic compounds are used in the state of solid or solution dispersed in a liquid. The organic electrochromic compounds are used in the state of solution solved in a liquid.
The electrochromic compound (hereinafter referred to simply as "EC compound") is further divided into two kinds according to its state in which it is colored, i.e. an EC compound coloring in a reduced state and an EC compound coloring in an oxidized state. The EC compound coloring in a reduced state is colored in a reduced state and is turned to colorless and transparent or to a different light color when it is oxidized. The EC compound coloring in an oxidized state is colored in the oxidized state and is turned to coloress and transparent or to a different light color when it is reduced.
Thus, the EC compound presents different color according to its state. Therefore, it is possible to make a display device by putting the EC compound in a cell having oppositely disposed electrodes at least one of which is transparent and applying a voltage across the cell. The color observed from outside on the transparent electrode changes in the pattern of the electrode when the voltage application across the cell is turned on or off.
There has been proposed to provide a diffusion type EC display device in a copending U.S. Patent Application Ser. No. 769,564. The diffusion type EC display device is an EC display device composed of a pair of oppositely disposed electrode plates at least one of which is transparent, and an EC composition sealed between the pair of electrode plates. The EC composition is composed of an EC compound coloring in a reduced state and a solvent. The EC compound coloring in a reduced state is sealed between the electrode plates in a reducing atmosphere. The solubility of the EC compound to the solvent is 10.sup.-2 mol/(100 g solvent) or more in a reduced state and 10.sup.-1 mol/(100 g solvent) or more in an oxidized state. The solvent is preferably composed of water and at least 70% by weight of alcohol. The alcohol is desirably polyhydric alcohol.
In operation of the diffusion type EC display device, the EC compound coloring in a reduced state is sealed in a cell in a reduced state. The diffusion type EC display device presents a colored pattern when no electric field is applied thereto. When an electric field is applied, the EC compound changes it color to the colorless and transparent state whereby a white color of emulsion or a color of a pigment appears on the anode upon application of an electric field. The anode is formed in a pattern to be displayed so that the change in color from the color of the EC compound in a reduced state to the color of pigment or emulsion is displayed in the form of the pattern. Upon turning off of the electric field, the oxidized EC compound immediately diffuses into the solvent and accordingly the displayed pattern quickly disappears.
The diffusion type EC display device is advantageous over the conventional "memory type" EC display device in that the display can be conducted by use of a lower voltage and the life is long. The "memory type" EC display device means the conventional EC display device using an EC composition in which the EC compound is hard to diffuse and accordingly the turn on and off of the display is carried out by applying a positive and negative potential to the cell. That is, when the display is to be provided, a positive potential is applied, and when the display is to be erased a negative potential is applied.
In contrast to the memory type display device, in the diffusion type display device the display can be turn on and off by simply turning on and off the application of the voltage across the cell. In the diffusion type EC display device, however, the fall time is markedly long in comparison with the rise time. The fall time is as long as several hundreds milliseconds to severals seconds, while the rise time is about 100 to 400 milliseconds. Therefore, the diffusion type EC display device cannot be used as a display in which a quick response is required such as a second display in a digital watch.